4.4.2. X-ray images of clusters and the morphology of the
intracluster gas

A tremendous increase in our understanding of the distribution of the X-ray
emitting gas in clusters has come about through imaging of the
two-dimensional
X-ray surface brightness. The launch of the Einstein X-ray
observatory satellite
made it possible to image X-ray clusters routinely, although a
rocket-borne X-ray
mirror was used to image the X-ray emission from the Coma, Perseus,
and M87/Virgo clusters prior to the launch of Einstein
(Gorenstein et
al., 1977,
1978,
1979).
The results of the X-ray imaging observations of
clusters have been extensively reviewed recently by
Forman and Jones (1982).

Forman and Jones (1982;
also Jones et
al., 1979,
and Jones and Forman
1984)
propose a two-dimensional classification scheme for the X-ray morphology of
galaxies, which they relate to the evolutionary state of the cluster as
determined by its optical properties
(Section 2.9,
2.10). This scheme
is presented in Table 3, which is
taken in large part from
Forman and Jones (1982).
In Figure 17 the X-ray surface brightness
distributions
from the Imaging Proportional Counter (IPC) on the Einstein X-ray
observatory are shown for representative examples of clusters of each
morphological type. The X-ray brightness distribution is shown as a contour
plot, in which the lines are loci of constant X-ray surface brightness.
The contours are superimposed on optical photographs of the cluster for
comparison.

First of all, Forman and Jones classify the clusters as being irregular
('early')
or regular ('evolved'), based on their overall X-ray distribution. The
early clusters
have irregular X-ray surface brightnesses, and often show small peaks in the
X-ray surface brightness, many of which are associated with individual
galaxies
in the cluster. Their X-ray luminosities Lx and X-ray
spectral temperatures Tg
are low. Optically, these clusters tend to be irregular clusters
(Table 1). That is,
they have irregular galaxy distributions with subclustering and with low
central concentration. They are not generally very rich, and tend to be
Bautz-Morgan types II to III and Rood-Sastry types F and I. They generally
have low velocity dispersions
r. They are
often, but not always, spiral rich.

On the other hand, the evolved clusters have regular, centrally condensed
X-ray structures
(Forman and Jones,
1982).
The X-ray distribution is smooth,
and X-ray emission peaks are not found associated with individual galaxies,
except possibly with a central dominant galaxy at the cluster
center. The evolved
clusters have high X-ray luminosities and gas temperatures. Optically,
these are
regular, symmetric clusters, generally of Bautz-Morgan type I to II and
Rood-Sastry types cD, B, L, or C. They are rich and have a high central
concentration of galaxies. They have high velocity dispersions and are
spiral poor in their galaxy composition.

The second determinant of the X-ray morphology of clusters is the presence
or absence of a central, dominant galaxy in the cluster. The X-ray
emission from
a cluster tends to peak at the position of such a galaxy. Clusters
containing such central dominant galaxies are classified by
Forman and Jones (1982)
as X-ray
dominant (XD); those without such a galaxy are classified as non-X-ray
dominant (nXD). The nXD clusters have larger X-ray core radii
rx
500 / h50
kpc. There is no strong X-ray emission associated with any individual
galaxy in these systems.

The XD clusters have small X-ray core radii rx 250 /
h50 kpc. The X-ray
emission is strongly peaked on the central dominant galaxy. In many cases,
spectral observations indicate that gas is cooling in this central peak
and being accreted onto the central galaxy
(Sections 4.3.3 and
5.7). The data of
Jones and Forman (1984)
suggest that all cooling flows are centered on the dominant
galaxies in XD clusters. Regular XD clusters have, on average, the
highest X-ray luminosities of any clusters
(Forman and Jones, 1982;
Jones and Forman, 1984).

Forman and Jones (1982)
and Jones et al.
(1979)
argue that this classification
scheme represents a sequence of cluster evolution, just as the sequence
of cluster
optical morphology (Section 2.5) was
related to the dynamical evolution
of the cluster in Section 2.9.
Specifically, the evolution of the overall
cluster distribution may be due to violent relaxation during the collapse
of the cluster. This occurs on the dynamical time scale of the cluster,
which depends only on its density (equation 2.32). On the other hand, the
formation of central dominant galaxies may be due to mergers of galaxies; as
demonstrated in Section 2.10.1,
this mechanism favors compact but poor
regions. Thus, while both a regular overall distribution of a cluster
and the
presence of a central dominant galaxy may indicate that the cluster has
undergone dynamical relaxation, the processes are different and depend in
different ways on the size and mass of the region. This provides a possible
qualitative explanation for this two-dimensional classification system
of X-ray morphology.

Figure 18. The X-ray surface brightness in
four double clusters, from
Forman et al.
(1981).
Contours of constant X-ray surface brightness are
shown superimposed on optical images of the clusters.

These double systems may represent an intermediate stage in the evolution
of clusters from irregular to regular distributions
(Tables 1 and
3). In
fact, numerical N-body simulations of cluster formation often show an
intermediate bimodal stage to the galaxy distributions (see
Figure 5c;
White, 1976c;
Ikeuchi and Hirayama,
1979;
Carnevali et al.,
1981).
With the
current (rather poor) statistics, the fraction of clusters detected in this
double phase ( 10%) is
consistent with the relatively short lifetime of the phase.

Figure 19. The X-ray surface brightness of
the Coma cluster of galaxies
from the
IPC on Einstein, kindly provided by Christine Jones and Bill
Forman. Contours
of constant X-ray surface brightness are shown superimposed on the optical
image of the cluster.

In the irregular and double clusters, the gas distribution is correlated
with the
galaxy distribution. The gas is probably roughly in hydrostatic
equilibrium with the cluster gravitational potential
(Section 5.5), which is primarily
influenced by
the distribution of the dynamically dominant missing mass component
(Section 2.8).
The fact that the X-ray surface brightness and galaxy distribution correlate
suggests that the galaxies and the missing mass have a similar distribution
in clusters; other evidence favoring this viewpoint was given in
Section 2.8.